Motor vehicles with internal combustion engines conventionally employ a fan to move cooling air through a radiator to reduce the temperature of coolant fluid circulated therein. For improved efficiency, a clutch is generally included that engages when the engine coolant is above a certain preselected temperature, and disengages when the coolant is below that temperature. Often, the clutch utilizes the viscous properties of a contained fluid to provide increased angular fan acceleration. A typical viscous clutch design utilizes fluid that is carried in a reservoir and is introduced to a working chamber, engaging the clutch and rotating the attached fan. The clutch drives the fan at approximately the input speed when additional cooling is required and allows the fan to rotate at a low disengaged speed when additional cooling is not required. The clutch is generally engaged by a thermostat that responds to air flowing through the radiator that is at a temperature representative of the engine's coolant temperature. The viscous clutch can generally only be engaged or disengaged, and it is engaged at or near engine speed, even when the cooling requirements could be supplied at a lower speed.
Coupling devices, including clutches for use in environments such as cooling systems, that operate with electro-magnetomechanical engagement mechanisms are known wherein rotation of an output member relative to a driven input member is controlled by means of magnetic flux. The magnetic flux lines pass through the input and output members and through air or fluid gaps to transfer torque. Fluids have been developed that free the input and output members from one another permitting relative and independent rotation therebetween when the magnetic field was absent. When the necessary electromagnetic coil is energized, the magnetic fluid locks the input and output members together for unitary rotation. Generally, this means that the fan is nearly instantaneously accelerated to engine speed, which results in the impartation of high stresses and tends to generate undesirable noise levels. Additionally, the electrical current is generally applied to the coil at a single engagement effective rate, whenever it is desirable to drive the fan.
Fluids for use in electro-magnetomechanical force transmission devices are known wherein material such as finely powdered iron or iron alloy particles are carried in a liquid such as mineral oil or silicone. The suspension of ferromagnetic particles in a fluid medium is known to affect the rheology of the fluid, particularly when subjected to magnetic flux. More specifically, flow characteristics can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. Such magnetorheological fluids have been found to have desirable electro-magnetomechanical interactive properties for advantageous use in variable power transmission devices such as clutches, where magnetic fields provide the essential coupling in the energy transfer process. For example, magnetorheological fluids exhibit a self reversing increase in viscosity when subjected to an increase in an externally supplied magnetic field. The common magnetic field is distributed throughout a region of space, generally in a variable manner depending on distance from the field's source with the flux density being high close to the surface of the source. The fluid's rheology varies depending on the magnetic flux density and the location in the magnetic field.
Actual application of magnetorheological fluid technology in uses such as vehicle cooling fan clutches has been slow to develop. A practical and competitive design has been elusive and therefore, a need continues to exist.